1. Field of the Invention
The present invention relates to a polarization analyzer for use in fields of optical communication such as wavelength division multiplexing (WDM) and optical measuring, and more particularly, to a polarization analyzer which uses a plurality of Faraday rotators to measure the polarization state of incident light with high precision and high speed.
2. Description of the Related Art
As is generally known, the overall communication speed of wavelength division multiplexing (WDM) communication is increasing at a high speed year after year due to application of broadband to the communication speed of each of the channels, and also due to the widening of the space between the channels.
As broadband application increases, and the widening of the space between the channels progresses, cross-talk between the channels in WDM has become a problem.
A polarization interleave method in which the light diffused to all the channels is subjected to linear polarization, and the planes of polarization between the adjacent channels are shifted by 90° with respect to each other, has been proposed as a method for reducing cross talk in this type of WDM communication, and development of this method is currently being carried out.
In WDM communication using this polarizing interleave method, it is necessary for the polarization properties of the optical components and the optical transmission paths to be accurately determined in advance.
The polarization properties of the optical components and the like for use in WDM communication is determined by checking the polarization of incident light and the polarization of emitted light and by measuring the changes in these polarizations.
A space analysis method and a time analysis method are two methods generally used as methods for measuring the polarization state of light.
As shown in FIG. 3, the space analysis method is a method in which incident light 300 is split and each light is subjected to a device having the known polarization properties, and the Stokes parameters S0, S1, S2 and S3 which indicate the polarization state of the incident light 300 is estimated from this amount of light.
That is to say, in FIG. 3, the incident light 300 is split in to four light beams which are light beam 304, light beam 305, light beam 306, and light beam 307 using beam splitter 301, beam splitter 302, and polarizing beam splitter 303.
Here, after the light beam 304 is transmitted through a λ/4 plate 308 and a polarizer 309 having an orientation of 45 degrees, the light is received at an optical receiver 310.
Also, the light beam 305 is received at the optical receiver 312 after being transmitted with the polarizer 311 having an orientation of 45 degrees.
The light beam 306 and the light beam 307 which have been split with the polarizing beam splitter 303 are respectively transmitted through a 0° and 90° polarizer, and are respectively directly received by an optical receiver 313 and an optical receiver 314.
In addition, the Stokes parameters S0, S1, S2 and S3 which indicate the polarization state of the incident light 300 can be estimated from the light amounts of each receiving signal of each of the optical receiver 310, the optical receiver 312, the optical receiver 313, the optical receiver 314.
Meanwhile, as shown in FIG. 4, the time analysis method is a method in which incident light 400 is transmitted in sequence through a λ/2 plate 401, a λ/4 plate 402 and a polarizer 403, and by measuring the amount of light transmitted at an optical receiver 404, the Stokes parameters S0, S1, S2 and S3 which indicate the polarization state of the incident light 400 can be estimated.
At this time, the orientation angle of the λ/2 plate 401, the λ/4 plate 402 and the polarizer 403 must be carefully measured a number of times, and the Stokes parameters S0, S1, S2 and S3 which indicate the polarization state of the incident light 400 can be estimated from the relationship between each orientation angle and the amount of light transmitted.
Incidentally, in the space analysis method, because the incident light can be subjected to a plurality of polarization rotations simultaneously, and the Stokes parameters S0, S1, S2 and S3 which indicate the polarization state of the incident light 300 can be measured speedily.
However, in this space analysis method, because accurately measuring the divergence ratio at the time of divergence of the light is difficult, the Stokes parameters S0, S1, S2 and S3 which are obtained have poor accuracy, and also the amount of light received by each one of the optical receivers is decreased because of the divergence of the light, and there is the problem that there is a tendency for measurement sensitivity to be reduced.
Meanwhile, in the time analysis method, because there is no light divergence, the measuring sensitivity is good, but because it uses a method in which the wavelength plate and the polarizer are mechanically rotated, this mechanical rotation takes time. Therefore, there is the problem that speedy measurement of the Stokes parameters S0, S1, S2 and S3 which indicate the polarization state of the incident light 400 is difficult.